The invention relates to a process and apparatus for the extraction of oil or polychlorinated biphenyl (referred to hereinafter as PCB) from a dielectric liquid containing oil or PCB by means of a highly volatile solvent, and for the distillation of the solvent from the mixture of the solvent and the oil or PCB.
So-called askarel transformers and capacitors are known, in which a liquid dielectric containing a PCB is used. Because this non-combustible PCB is difficult to decompose biologically and because in an environmental fire within a certain temperature range highly toxic polychlorodibenzofurans and polychlorodibenzodioxins may be formed by the pyrogenic decomposition of the PCB, equipment of this type must be replaced within a short period of time or filled with a harmless dielectric replacement liquid.
To achieve this, EP 0147860 discloses, for example, a process for the replacement of a PCB by a high boiling dielectric liquid free of PCB. This process is applied, for example, in the case of an induction installation, in which the electric coil has been immersed with a porous, cellulose-like insulation into a container of an impregnating liquid containing PCB. Initially, the major portion of the impregnating liquid containing PCB is drained from the container. A dielectric liquid free of PCB is then filled into the container as an intermediate medium; this liquid being miscible with PCB and having an adequately low viscosity so as to flow in the container and penetrate into the interstices of the porous electric insulation. The inductive installation is next activated for a period of time sufficient to wash the PCB from an impregnated medium containing PCB within the porous insulation and absorb it into the intermediate medium. Subsequently, the intermediate medium containing the PCB washed from the container is drained. These process steps are repeated until the insulation is adequately cleaned, whereupon a permanent liquid free of PCB is filled into the container so that the electrical installation may be classified as free of PCB.
In oil-impregnated equipment, for example transformers, the insulation contains oil. In a condensation drying apparatus according to DE 27 56 298 (U.S. Pat. No. 4,292,744), not only is water removed from the insulation during drying, but in addition, the higher boiling liquid, for example oil, is also removed. The transformer to be cleaned in an autoclave is heated by means of the condensation heat of a solvent. During the heating, the solvent becomes mixed with the oil which is dissolved out of the insulation.
As the PCB or oil from the electric apparatus to be cleaned dissolves in the solvent present in the autoclave (container), it has been necessary heretofore to drain the oil or PCB-solvent mixture from the autoclave and to fill in a new solvent. This was done in order to assure optimal condensation of the solvent on the apparatus and thus to assure the dissolution of the higher boiling PCB's or oils from the apparatus, particularly from its insulation. In addition, the mixture drained off had to be regenerated by distillation.
An installation for the heating and drying of paper-insulated electro-technical parts according to DE 30 38 493 is also known, in which the low and high boiling components of the contaminated heating liquid are separated, and wherein for the evaporation of the heating liquid and for the separation of its vapor from high boiling components, a thin film evaporator is used.
A disadvantage of this installation consists in particular of the fact that a secondary evaporator is required in addition to the evaporator, with both of them being located outside the drying boiler.